When Admixtures Turn Us into Chemists
Admixture: ingredients (other than water, aggregate, hydraulic cement, and fiber reinforcement) added to a batch of concrete or mortar before or during its mixing.
Chemical admixtures are now a routine part of concrete work. They react with the components inside the slab, and the installer’s goal is the same as it has always been: high-quality concrete with shorter setting times.
However, chemical reactions are a chemist’s world. Admixtures play an increasingly prominent role in concrete installation, and that makes chemists out of building contractors.
- “How Beneficial Is an Admixture?”
- The Influence of Relative Humidity (RH)
- Moisture Measurement
- Additional Reading
“How Beneficial Is an Admixture?”
The answer is changing as quickly as the new admixture ingredients. However, we can generally observe the main effects of those most widely applied to concrete.
Superplasticizers are popular in admixtures which aim to reduce moisture content (MC) in concrete slabs. This, in turn, speeds drying time. However, the various ingredients of superplasticizers produce different results in concrete MC.
Superplasticizer admixtures containing lignosulfonate are successful in lower doses. Higher doses actually slow concrete set times and can introduce unintended air into the mix, which reduces strength. That is not the same as air entrainment, the deliberate addition of microscopic air cells to protect concrete against freeze-thaw damage. Lignosulfonate hybrid mixes result in variable compressive strength readings in concrete slabs.
Superplasticizer admixtures produced with polymer compounds also produce mixed results. If the main goal is to more efficiently disperse MC, polymers with lower molecular weight tend to outperform admixtures with higher molecular weight.
Many other factors influence concrete installation: cement composition, its alkali content, and particle distribution to name a few. Ultimately, admixtures are only successful when they assist in the equilibration of concrete MC. While admixtures can speed the curing process, they do not always benefit the drying process of a concrete slab. It’s only as water vapor migrates from the bottom of the slab to the surface that concrete dries. That is the goal.
The Influence of Relative Humidity (RH)
Changes in RH cause concrete to absorb and release water vapor, and this alters drying times. Admixtures may provide a chemical means of speeding up moisture dispersal, but at a chemical cost to the concrete itself. If concrete has not reached equilibrium moisture content (EMC), slab performance and longevity are compromised.
Chemical admixtures may also be incompatible with flooring applications. The pH levels of concrete can break down flooring adhesives, and incompatible admixtures can further complicate matters. It is wise to investigate all thoroughly before attempting such feats of flooring chemistry.
Moisture Measurement
Concrete MC can be measured to adapt for changes to RH. Wagner Meters has developed the Rapid RH® in situ test method for testing in compliance with ASTM International Standard F2170.
It may not resolve the technical evolution of admixtures, but MC management remains at the core of every successful concrete slab installation.
Free Download: 4 Reasons Why Your Concrete Is Taking Forever to Dry
Additional Reading
ACI Education Bulletin E4-12, Chemical Admixtures for Concrete. Farmington Hills, MI: American Concrete Institute.
Ramachandran, V.S. (Ed.) Concrete Admixtures Handbook: Properties, Science, and Technology, 2nd Ed. Park Ridge, NJ: Noyes Publications, 1995.
Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.
Last updated on September 22nd, 2026
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